This paper designs beamforming for a wideband, multi-user, near-field downlink in which a Reconfigurable Refractive Surface (RRS) serves as the transmit antenna. An RRS is a subwavelength metasurface whose elements refract and phase-shift signals using low-power diodes, replacing the power-hungry phase shifters of conventional phased arrays. At high carrier frequencies and large apertures, users fall in the RRS near field and the RRS becomes frequency selective, so different subcarriers focus at different locations and lose focus - the beam-split effect - which here is worse because near-field geometry and RRS frequency selectivity act together. The authors introduce a Delayed-RRS structure that adds true time-delay units to the elements, model the frequency-selective transmission coefficient, and formulate a max-min data-rate problem jointly optimizing the digital beamformer, the RRS phase shifts, and the time delays, solved by an alternating-optimization algorithm. Simulations show that jointly accounting for the near-field condition and the RRS frequency selectivity is necessary and quantify the resulting data-rate gains. Part of the work was accepted at IEEE WCNC 2025.
This paper designs beamforming for a wideband, multi-user, near-field downlink in which a Reconfigurable Refractive Surface (RRS) serves as the transmit antenna. An RRS is a subwavelength metasurface…